1. Field of the Invention
This invention relates to a contact switching arrangement with a flat, electrically non-conducting cover, which forms a switching area on a top facing the user, and has an underside facing away from the user, on which at least one electrode is arranged and is connected with a switching arrangement for triggering a contact signal when a finger of the user, or similar conducting body, approaches the switching area.
2. Discussion of Related Art
Capacitive contact switches, operating on the basis of electric field effects, for example, are known in the prior art, such as described in U.S. Pat. No. 5,594,222 or European Patent Reference EP 0 859 467 A1.
Such contact switches are generally employed for trigger switching processes through a continuous surface made of glass or plastic. For this purpose the contact switches are arranged behind the surface which, for example, represents the control panel of an electrical or electrically controllable device, a cooking surface made of glass ceramic material, or other input unit. The switching process is then triggered by touching the surface near the contact switch, wherein the actual switching function is performed by electronic switches or relays.
The contact switches based on electrical field effects, or the capacitive contact switches generally have an electrode arranged behind a continuous cover near the switching area. The switching function is triggered if a conducting body of sufficient size, for example a human finger, is near the electrode.
In contact switches common in the prior art, two electrodes are located on the side of the cover facing away from the user. A possibly transparent electrode is arranged on the side facing the user, and covers the two lower electrodes. The lower electrodes are controlled by an electronic device so that they act as series connection of two capacitors. In this case the coupling is provided via the upper electrode. If the user touches the upper electrode, it is connected to ground and the coupling of the signals is significantly weakened.
A contact switch is also known from the prior art, on which two electrodes are arranged next to each other only on an underside of the cover. The two electrodes are controlled so that an electrical alternating field is built up which penetrates the continuous cover. The electrical alternating field is disturbed when touching the switching area. Such a disturbance can then be detected by a suitable electrical circuit.
The cover, or area, must not be electrically conductive in this case, or else the electrical fields being built up would be shielded. Thus the cover represents a non-conductor.
For design reasons, the control panels of electronic devices in particular are often formed as metallic surfaces. Such metals are generally not transparent to electromagnetic radiation, and thus it is shielded by the metal. Thus the known contact switches cannot be actuated through closed metallic surfaces. This also applies to metallized surfaces of glass or transparent plastic, which also act as shields against electromagnetic radiation.
A foil which is transparent to electrical fields is offered by the 3M Company under the name “Radiant Mirror Film”. Such a film or foil has several polymer layers. With a smooth embodiment of the foil, more than 95% of visible light in the wave range of 400 to 700 nm are mirror-reflected at an angle of incidence of 0 to 90°. The structure and function of such a polymer film is described in European Patent Reference EP 0 962 807 A2, for example. Here, an especially great light reflection is achieved by a layer structure of double-refracting organic materials, which lends a metallic mirrored appearance to the film. The film is non-conductive and thus is suited as a non-conductor with capacitive contact switches, or those based on electrical field effects.